Get discard support upstreamed

[g2p]

Currently mirage-block doesn't expose discard in type signatures. There's been a PR providing it, check its merge/release status. Otherwise we have to stub it out which is bad for usability.

[g2p]

Just expose the signature that's been merged in mirage-block-unix:

val discard: t -> int64 -> int64 -> (unit, write_error) result io
(** [discard sector n] signals that the [n] sectors starting at [sector]
    are no longer needed and the contents may be discarded. Note the contents
    may not actually be deleted: this is not a "secure erase". *)

[g2p]

Related Solo5 issue for discard: Solo5/solo5#329